1. Field of the invention
The present invention relates to a self-luminous display device having, in each pixel circuit, a light-emitting diode adapted to emit light when applied with a bias voltage, a drive transistor adapted to control a drive current flowing through the light-emitting diode and a holding capacitor coupled to a control node of the drive transistor, and to a driving method of the same.
2. Description of the Related Art
An organic electro-luminescence element is known as an electro-optical element used in a self-luminous display device. This element, typically referred to as an OLED (Organic Light Emitting Diode), is a type of light-emitting diode.
The OLED has a plurality of organic thin films stacked one atop another. These thin films function, for example, as an organic hole transporting layer and organic light-emitting layer. The OLED is an electro-optical element which relies on the light emission of an organic thin film when applied with an electric field. Controlling the current level through the OLED provides color gray levels. Therefore, a display device using the OLED as an electro-optical element has, in each pixel, a pixel circuit which includes a drive transistor and capacitor. The drive transistor controls the amount of current flowing through the OLED. The capacitor holds the control voltage of the drive transistor.
Various types of pixel circuits have been proposed to date.
Chief among the proposed types of circuits are the 4T1C pixel circuit with four transistors (4T) and one capacitor (1C), 4T2C, 5T1C and 3T1C pixel circuits.
All of the above pixel circuits are designed to prevent image quality degradation resulting from the variation in transistor characteristics. The transistors are made of TFTs (Thin Film Transistor). These circuits are intended to maintain the drive current in the pixel circuit constant so long as a data voltage is constant, thus providing improved uniformity across the screen (brightness uniformity). The characteristic variation of the drive transistor, adapted to control the amount of current according to the data potential of an incoming video signal, directly affects the light emission brightness of the OLED particularly when the OLED is connected to power in the pixel circuit.
The largest of all the characteristic variations of the drive transistor is that of a threshold voltage. A gate-to-source voltage of the drive transistor must be corrected so as to cancel the effect of the threshold voltage variation of the drive transistor from the drive current. This correction will be hereinafter referred to as a “threshold voltage correction.”
Further, assuming that the threshold voltage correction will be performed, further improved uniformity can be achieved if the gate-to-source voltage is corrected so as to cancel the effect of a driving capability component (typically referred to as a mobility). This component is obtained by subtracting the components causing the threshold variation and other factors from the current driving capability of the drive transistor. The correction of the driving capability component will be hereinafter referred to as a “mobility correction.”
The corrections of the threshold voltage and mobility of the drive transistor are described in detail, for example, in Japanese Patent Laid-Open No. 2006-215213 (hereinafter referred to as Patent Document 1).